Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, a one-dimensional array typically has a multiplicity of transducer elements arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are delayed and summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate time delay (and/or phase shift) and gain to the signal from each receiving transducer.
FIG. 1A depicts an ultrasound imaging system consisting of four main subsystems: a beamformer 2, processors 4 (including a separate processor for each different mode), a scan converter/display controller 6 and a kernel 8. System control is centered in the kernel, which accepts operator inputs through an operator interface 10 and in turn controls the various subsystems. The master controller 12 performs system level control functions. It accepts inputs from the operator via the operator interface 10 as well as system status changes (e.g., mode changes) and makes appropriate system changes either directly or via the scan controller. The system control bus 14 provides the interface from the master controller to the subsystems. The scan control sequencer 16 provides real-time (acoustic vector rate) control inputs to the beamformer 2, system timing generator 24, processors 4 and scan converter 6. The scan control sequencer 16 is programmed by the host with the vector sequences and synchronization options for acoustic frame acquisitions. The scan converter broadcasts the vector parameters defined by the host to the subsystems via scan control bus 18.
The main data path begins with the analog RF inputs to the beamformer 2 from the transducer 20. The beamformer 2 outputs data to a processor 4, where it is processed according to the acquisition mode. The processed data is output as processed vector (beam) data to the scan converter/display controller 6. The scan converter accepts the processed vector data and outputs the video display signals for the image to color monitor 22.
Referring to FIG. 1B, a conventional ultrasound imaging system includes a transducer array 24 comprised of a plurality of separately driven transducer elements 26, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter (not shown). The ultrasonic energy reflected back to transducer array 24 from the object under study is converted to an electrical signal by each receiving transducer element 26 and applied separately to beamformer 2.
The echo signals produced by each burst of ultra-sonic energy reflect from objects located at successive ranges along the ultrasonic beam. The echo signals are sensed separately by each transducer element 26 and the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range. Due to the differences in the propagation paths between an ultrasound-scattering sample volume and each transducer element 26, however, these echo signals will not be detected simultaneously and their amplitudes will not be equal. Beamformer 2 amplifies the separate echo signals, imparts the proper time delay to each, and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from the sample volume. Each beamformer channel 28 receives the analog echo signal from a respective transducer element 26.
To simultaneously sum the electrical signals produced by the echoes impinging on each transducer element 26, time delays are introduced into each separate beamformer channel 28 by a beamformer controller 30. The beam time delays for reception are the same delays as the transmission delays. However, the time delay of each beamformer channel is continuously changing during reception of the echo to provide dynamic focusing of the received beam at the range from which the echo signal emanates. The beamformer channels also have circuitry (not shown) for apodizing and filtering the received pulses.
The signals entering the summer 32 are delayed so that when they are summed with delayed signals from each of the other beamformer channels 28, the summed signals indicate the magnitude and phase of the echo signal reflected from a sample volume located along the steered beam. A signal processor or detector 34 converts the received signal to display data. In the B-mode (grey-scale), this would be the envelope of the signal with some additional processing such as edge enhancement and logarithmic compression. The scan converter 6 receives the display data from detector 34 and converts the data into the desired image for display. In particular, the scan converter 6 converts the acoustic image data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on display monitor 22, which images the time-varying amplitude of the envelope of the signal as a grey scale.
A phased-array ultrasound transducer consists of an array of small piezoelectric elements, with an independent electrical connection to each element. In most conventional transducers the elements are arranged in a single row, spaced at a fine pitch (one-half to one acoustic wavelength on center). As used herein, the term "1D" array refers to a single-row transducer array having an elevation aperture which is fixed and a focus which is at a fixed range; the term "1.5D" array refers to a multi-row array having an elevation aperture, shading, and focusing which are dynamically variable, but symmetric about the centerline of the array; and the term "2D" array refers to a multi-row transducer array having an elevation geometry and performance which are comparable to azimuth, with full electronic apodization, focusing and steering. Electronic circuitry connected to the elements uses time delays and perhaps phase rotations to control the transmitted and received signals and form ultrasound beams which are steered and focused throughout the imaging plane. For some ultrasound systems and probes, the number of transducer elements in the probe exceeds the number of channels of beamformer electronics in the system. In these cases an electronic multiplexer is used to dynamically connect the available channels to different (typically contiguous) subsets of the transducer elements during different portions of the image formation process.
A typical 1D linear or convex transducer array and multiplexer is shown schematically in FIG. 2. The beamformer 2 has 128 beamformer channels, but the transducer array 24 has significantly more elements (typically 192 to 256). The multiplexer 36 allows any set of up to 128 contiguous transducer elements 26 to be simultaneously connected to the beamformer channels 28 via coaxial cable bundles 38. By closing switches connected to elements 0 through 127, the beamformer 2 is connected to the left end of the transducer array and focused beams of ultrasound may be transmitted and received to acquire data for the corresponding edge of the image. As the point of origin of successive ultrasound beams steps along the transducer array 24 to the right, it becomes advantageous to shift the active aperture so that the origin of the ultrasound beam is centered within it. To shift the aperture from the extreme left end of the array by one element toward the right, the multiplexer switch connected to element 0 is opened and the switch connected to element 128 is closed. This shifts beamformer channel 0 from the left end to the right end of the active aperture, while leaving all other channels and elements connected as before. The time delays and other beamforming parameters are changed by the software to correspond to the new multiplexer state and one or more additional image vectors are acquired. Then the aperture is stepped further to the right, by opening the switch connected to element 1 and closing the switch connected to element 129, leaving the multiplexer 36 in the state shown in FIG. 2. In this manner the active aperture can be stepped sequentially from one end of the transducer array 24 to the other. Alternatively, the same multiplexer hardware may be used to scan the active aperture more rapidly across the array by switching several transducer elements per step. In some imaging modes, successive apertures may be selected non-sequentially, jumping back and forth between the left and right ends of the transducer array.
The spatial resolution, along the lateral axis, of a conventional ultrasound imaging system is determined by the imaging F number and the operating wavelength. A small F number, used for high-resolution imaging, requires a large aperture. The maximum aperture size is limited to the product of the element pitch and the number of channels available on the beamformer. There is a need for a technique that overcomes this limitation on maximum aperture size.